This invention relates to a motor-generator set for producing, within a predetermined range, a constant volts/hertz output from the generator.
Motor-generator sets are known which comprise an engine (motor), such as a diesel engine, driving an AC generator. The speed of the engine is controlled by a governor to run at a constant, but adjustable, speed so that the generator produces an output voltage at, say, 60 Hertz. A voltage regulator is normally provided to control the output voltage of the generator. When the motor-generator set is used to drive a synchronous motor, it is sometimes desirable to alter the output frequency of the generator in order to vary the speed of the synchronous motor being powered by the motor-generator set. This might be done for "soft" starting of motors or to derive increased power from the electric motor. However, if the frequency of the current provided to the motor is reduced, the applied voltage should also be reduced, because the back emf of the motor is reduced, resulting in increased current drain which could damage the motor or the generator.
Electronic controllers are known which can simultaneously alter frequency and voltage from the generator, but they are relatively complex.
U.S. Pat. No. 3,751,192 of Boyd, issued Aug. 7, 1973, discloses a generator driven by a constant torque arrangement. The generator is specially constructed to produce constant volts/hertz in operation and powers an electric motor which drives a pump. As the viscosity of the fluid being pumped decreases, the motor tends to speed up and the constant torque drive increases the speed of the generator so that its frequency and voltage increase.